In reproduction, all environmental and endogenous cues ultimately converge upon the single common neuroendocrine pathway of gonadotropin-releasing hormone (GnRH) from the hypothalamus and its stimulation of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary. GnRH has traditionally been thought of as the first step in the reproductive cascade; its commanding role in this biological hierarchy allows it to control pulsatile gonadotropin secretion, modulate gonadal steroid feedback, and ultimately determine the initiation of pubertal development and fertility. In 2003, we reported that, in both the human and the mouse, mutations in GPR54 cause a failure of the normal patterns of pulsatile GnRH release and hypogonadotropic hypogonadism. The ligand of GPR54 is kisspeptin--the impact of kisspeptin/GPR54 on GnRH secretion is well conserved across mammalian species; it is a genetic determinant and indisputable gatekeeper of normal reproductive function. Kisspeptin is a powerful stimulus of GnRH secretion and is a vital agent for activating the hypothalamic pituitary gonadal axis. . This grant will now explore the physiology of the kisspeptin pathway using the human as the model organism. The first specific aim explores the protein expression of kisspeptin using carefully assembled hypothalamic specimens from individuals representing all phases of reproductive life, including the mini-puberty of infancy, the childhood quiescence, the pubertal transition, reproductive maturity, and menopause. The second specific aim utilizes kisspeptin to interrogate of the GnRH neuron in vivo. Administration of kisspeptin to meticulously phenotyped and genotyped patients with hypogonadotropic hypogonadism will allow assessments of the functional integrity of the GnRH neuron, including the properties of GnRH neuronal fate specification, peptide synthesis and release. The third specific aim employs diverse methods of kisspeptin administration to selectively manipulate the reproductive cascade (selective, reversible suppression or sustained stimulation) and explore its role as a potential therapeutic target.